1. Field of the Invention
The present invention relates to an MEMS sensor.
2. Description of Related Art
In recent years, an MEMS sensor such as an Si (silicon) microphone produced by MEMS (Micro Electro Mechanical Systems) has been employed as a microphone loaded on a portable telephone or the like.
FIGS. 5A to 5K are schematic sectional views successively showing the steps of producing a conventional Si microphone 101. The method of producing the conventional Si microphone 101 and the structure thereof are now described with reference to FIGS. 5A to 5K.
In order to produce the conventional Si microphone 101, SiO2 (silicon oxide) is deposited on the overall surfaces of an Si wafer W2 by thermal oxidation, as shown in FIG. 5A. Thus, a lower sacrificial layer 111 made of SiO2 is formed on the upper surface of the Si wafer W2. Further, an SiO2 film 119 is formed on the lower surface of the Si wafer W2.
Then, a photoresist film 120 having holes 121 of a prescribed pattern is formed on the upper surface of the lower sacrificial layer 111, as shown in FIG. 5B. The lower sacrificial layer 111 is etched through the photoresist film 120 employed as a mask, whereby a plurality of (four in FIG. 5C) recesses 112 are formed in the upper surface of the lower sacrificial layer 111, as shown in FIG. 5C. After the formation of the recesses 112, the photoresist film 120 is removed.
Then, polysilicon is deposited on the overall surfaces of the lower sacrificial layer 111 and the SiO2 film 119 by LPCVD (Low Pressure Chemical Vapor Deposition). The polysilicon film covering the lower sacrificial layer 111 is doped with phosphorus, and thereafter portions of this polysilicon film other than that present on a prescribed region including the plurality of recesses 112 are removed by well-known photolithography and etching. Thus, a thin-film polysilicon plate 104 is formed on the prescribed region of the lower sacrificial layer 111, as shown in FIG. 5D. Further, a polysilicon film 113 is formed on the SiO2 film 119.
Then, SiO2 is deposited on the overall surfaces of the lower sacrificial layer 111 and the polysilicon plate 104 by PECVD (Plasma Enhanced Chemical Vapor Deposition). Then, unnecessary portions of the deposited SiO2 film are removed by well-known photolithography and etching. Thus, an upper sacrificial layer 114 made of SiO2 is formed on the polysilicon plate 104 and a region around the same, as shown in FIG. 5E.
Then, polysilicon is deposited on the lower sacrificial layer 111, the upper sacrificial layer 114 and the polysilicon film 113 by LPCVD (Low Pressure Chemical Vapor Deposition). Thus, the polysilicon film deposited on the polysilicon film 113 and the polysilicon film 113 are integrated into a polysilicon film 115, as shown in FIG. 5F. On the other hand, the polysilicon film deposited on the lower sacrificial layer 111 and the upper sacrificial layer 114 is doped with phosphorus, and thereafter patterned by well-known photolithography and etching. Thus, a thin-film back plate 105 having a large number of holes 106 is formed on the upper sacrificial layer 114, as shown in FIG. 5F.
Then, a photoresist film 122 having holes 123 of a prescribed pattern is formed on the overall region of the upper sacrificial layer 114 including the back plate 105, as shown in FIG. 5G. Then, the upper sacrificial layer 114 is etched through the photoresist film 122 employed as a mask. Thus, a plurality of (four in FIG. 5H) recesses 117 are formed in the upper surface of the upper sacrificial layer 114 while unnecessary portions (other than the portion opposed to the upper sacrificial layer 114) of the lower sacrificial layer 111 are removed, as shown in FIG. 5H. After the formation of the recesses 117, the photoresist film 122 is removed.
Then, the polysilicon film 115 is removed, and an SiN (silicon nitride) film 107 is thereafter formed on the upper region of the Si wafer W2 by PECVD, as shown in FIG. 5I.
Then, holes 118 communicating with the holes 106 of the back plate 105 respectively are formed in the SiN film 107 by well-known photolithography and etching, as shown in FIG. 5J. Thus, the upper sacrificial layer 114 is partially exposed through the holes 106 and 118. Further, an opening is formed in the portion of the SiO2 film 119 opposed to the polysilicon plate 104 by well-known photolithography and etching. Then, the Si wafer W2 is etched through this opening, so that a through-hole 103 is formed in the Si wafer W2. Consequently, the lower sacrificial layer 111 is partially exposed through the through-hole 103.
Then, an etching solution capable of etching SiO2 is supplied through the through-hole 103 and the holes 106 and 118, to wet-etch the upper sacrificial layer 114 and the lower sacrificial layer 111. Thus, a cavity 124 of a small interval is formed between the Si wafer W2 and the polysilicon plate 104 and the polysilicon plate 104 floats up from the upper surface of the Si wafer W2, as shown in FIG. 5K. Further, a cavity 110 of a small interval is formed between the polysilicon plate 104 and the back plate 105, and the back plate 105 floats up from the upper surface of the polysilicon plate 104.
Thereafter the Si wafer W2 is divided into an Si substrate 102 of each device size, whereby the Si microphone 101 is obtained with the polysilicon plate 104 and the back plate 105 opposed to each other through the cavity 110. Portions of the SiN film 107 having entered the recesses 117 of the upper sacrificial layer 114 become protrusions 109 protruding toward the polysilicon plate 104, to function as stoppers for preventing the polysilicon plate 104 and the back plate 105 from adhesion and a short circuit. Further, portions of the polysilicon plate 104 having entered the recesses 112 of the lower sacrificial layer 111 become protrusions 108 protruding toward the upper surface of the Si wafer W2, to function as stoppers for preventing the Si substrate 102 and the polysilicon plate 104 from adhesion. The polysilicon plate 104 and the back plate 105 are supported by unshown wires.
The polysilicon plate 104 and the back plate 105 form a capacitor portion 125 opposed through the cavity 110. When a sound pressure (sound wave) is input in the Si microphone 101 from the through-hole 103, the polysilicon plate 104 and the back plate 105 vibrate due to this sound pressure (sound wave), and the Si microphone 101 outputs an electric signal responsive to a change of the capacitance of the capacitor portion 125 resulting from this vibration of these plates 104 and 105.
In the Si microphone 101, however, both of the polysilicon plate 104 and the back plate 105 vibrate due to the sound pressure (sound wave) input from the through-hole 103, and hence the input sound wave may resonate.
Further, the cavity 124 is formed between the Si substrate 2 and the polysilicon plate 104, and the cavity 110 is formed between the polysilicon plate 104 and the back plate 105, while the polysilicon plate 104 and the back plate 105 are supported by the unshown wires to be in the states floating in the air respectively. In the Si microphone 101, therefore, the structure of the capacitor portion 125 is complicated, and the shock resistance of the capacitor portion 125 is not sufficient.
In order to form the two cavities (110 and 124) in the capacitor portion 125, the two steps including the step of forming the lower sacrificial layer 111 (see FIG. 5A) and the step of forming the upper sacrificial layer 114 (see FIG. 5E) are required as those of forming sacrificial layers for forming the cavities. In order to reduce the time necessary for removing the upper sacrificial layer 114 and the lower sacrificial layer 111, further, the through-hole 103 is formed in the Si wafer W2 and the etching solution is supplied through the through-hole 103 and the holes 106 and 118, thereby progressing the etching of the lower sacrificial layer 111 in parallel with the etching of the upper sacrificial layer 114. In order to form the through-hole 103 in the Si wafer W2, however, an opening must be formed in the SiO2 film 119 provided on the lower surface of the Si wafer W2, for etching the Si wafer W2 from this opening. In other words, two steps including those of forming the opening in the SiO2 film 119 and etching the Si wafer W2 must unavoidably be added. Consequently, the steps of producing the Si microphone 1 are disadvantageously complicated.